Removing unused rt_OsEventObserver

[SenRamakri]

Description

This removes rt_OsEventObserver from RTX. It was added as part of uvisor and is not used and we already have RTX event mechanism to track any thread related events. This will save around 150 bytes, and this also helps remove unwanted code adding to CPU cycles on every thread switch.

Pull request type

[x] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[0xc0170]

As stated, not used, thus no dependency on uvisor removal that is currently pending ?

[SenRamakri]

@0xc0170 - Yes, this doesn't depend on uvisor removal. May be this was added for some debugging purposes. So, can be removed.

[deepikabhavnani]

rt_OsEventObserver.{c,h} | Added an interface for uVisor to be notified about certain events from privileged code

@SenRamakri - Got reference of EventObserver from old docs (not sure if still valid) https://docs.mbed.com/docs/mbed-os-handbook/en/latest/advanced/cmsis-rtos/

[SenRamakri]

@deepikabhavnani - Thanks for pointing that out. I'm looking into that, its still not clear if/how its used.

[deepikabhavnani]

@SenRamakri - As per https://github.com/ARMmbed/mbed-os/blob/master/tools/importer/cmsis_importer.json, below mentioned PR's are related to uVisior change and is pulled in everytime we do CMSIS update.

https://github.com/ARMmbed/mbed-os/commit/adb6e821cc448f005e63ba9c7e2ce335abd54082
https://github.com/ARMmbed/mbed-os/commit/acd8ab1ff0588410f0fc5463fa12094250a371fb
https://github.com/ARMmbed/mbed-os/commit/1a511c456143bd31606f6a47807f506da1b51b90
https://github.com/ARMmbed/mbed-os/commit/474d2e3c9944d2866a0643795aafdfee743488eb
https://github.com/ARMmbed/mbed-os/commit/7b3a3a441ab6451462bd476721d6e8cb6ddd6c9d

On approval of this PR, all the commits reverted should be removed from cmsis_importer.json as well, else we will have them back in next CMSIS update

[SenRamakri]

@deepikabhavnani - Thanks for finding the above commits. I'm waiting on @alzix to review this so that we can confirm if we can revert all of them.

[alzix]

please note that this PR can only be merged after #7592.
If we merge it before removing uVisor - uVisor will remain in broken stat, which is suboptimal.

[cmonr]

Thanks @alzix. Updated the PR to more better indicate this.

In the meantime, @bulislaw @alzix @deepikabhavnani would y'all mind taking a look at this PR?

[bulislaw]

+1 for removing that.

[deepikabhavnani]

As per https://github.com/ARMmbed/mbed-os/blob/master/tools/importer/cmsis_importer.json, below mentioned PR's are related to uVisior change and is pulled in everytime we do CMSIS update.

adb6e82
acd8ab1
1a511c4
474d2e3
7b3a3a4

All the above commits reverted should be removed from cmsis_importer.json as well, else we will have them back in next CMSIS update.

I would recommend to add a revert PR to above commits, instead of code change

[cmonr]

@SenRamakri Thoughts on deepikabhavnani suggestion on instead applying git reverts to this PR instead? I like the idea since it makes tracking a bit more obvious.

[deepikabhavnani]

This PR will not be needed after CMSIS update #7875

[0xc0170]

This PR will not be needed after CMSIS update #7875

Closing as not needed anymore