Tools: Remove second header when every bl config param is used

[theotherjimmy]

Description

Resolves #7190

The issue is simple: the managed bootloader mode will accept both
target.bootloader_img and target.restrict_size at the same time.
When combined with target.header_format though, you end up with
two application headers, one before the application and one after.

This PR removes the second one; the after-application header.

Pull request type

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

[cmonr]

Nice and simple change, with a good description to boot.

[cmonr]

Holding off on CI until the Test pipeline is a bit shorter.

[cmonr]

Getting this in the queue since we're restarting other paused tests.
/morph test

[mbed-ci]

Test : FAILURE

Build number : 2120
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7196/2120

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2351
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7196/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1975
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7196/

[mbed-ci]

Test : SUCCESS

Build number : 2137
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7196/2137