stdlib: Address a couple of ABI FIXMEs by deleting code

[slavapestov]

We have an implicit conversion to AnyHashable, so there's no
need to have the special subscript on Dictionary at all.

[slavapestov]

@swift-ci Please smoke test

[slavapestov]

@swift-ci Please smoke test

[slavapestov]

@nkcsgexi I removed a subscript in this patch, but swift-api-digester did not flag it. Do you mind taking a look?

[slavapestov]

@swift-ci Please smoke test

[nkcsgexi]

You are right, the api-digester seems to have trouble digest subscripts. Do you mind file a radar?

[slavapestov]

Sure :)

[DougGregor]

Looks like that test was checking for the now-removed _Hashable protocol

[slavapestov]

@nkcsgexi rdar://30646610 rdar://30646634

[slavapestov]

@swift-ci Please smoke test

[slavapestov]

@swift-ci Please smoke test

[nkcsgexi]

Saw them, thank you!

[slavapestov]

@swift-ci Please smoke test Linux

[slavapestov]

@ddunbar any ideas what's going on here?

******************** TEST 'llbuild-unit :: BuildSystem/BuildSystemTests/BuildSystemTaskTests.directorySignature' FAILED ********************
Note: Google Test filter = BuildSystemTaskTests.directorySignature
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from BuildSystemTaskTests
[ RUN      ] BuildSystemTaskTests.directorySignature
/home/buildnode/disk2/workspace/swift-PR-Linux-smoke-test/branch-master/llbuild/unittests/BuildSystem/BuildSystemTaskTests.cpp:167: Failure
Value of: resultA->toData() != resultB->toData()
  Actual: false
Expected: true
[  FAILED  ] BuildSystemTaskTests.directorySignature (3 ms)
[----------] 1 test from BuildSystemTaskTests (3 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (4 ms total)
[  PASSED  ] 0 tests.
[  FAILED  ] 1 test, listed below:
[  FAILED  ] BuildSystemTaskTests.directorySignature

 1 FAILED TEST

Public CI is hitting the same issue: https://ci.swift.org/job/oss-swift-incremental-RA-linux-ubuntu-16_10/2054/consoleFull#-1082042515fca400bf-2f4a-462e-b517-e058d770b2d7

[ddunbar]

@slavapestov just saw that, a change exposed a problem on 16.04 I need to investigate -- I just reverted for now

[ddunbar]

@swift-ci please smoke test linux

[slavapestov]

@ddunbar Thank you!