Remove unused ENABLE_DILITHIUM CMake option

[andrewhop]

Description of changes:

Dilithium (really ML-DSA) is always enabled in AWS-LC now. No C or C++ files used the ENABLE_DILITHIUM define.

Call-outs:

Any customers or automated builds of AWS-LC that define the CMake option will not be broken by this. They will get a small message and the build will continue:

CMake Warning:
  Manually-specified variables were not used by the project:

    ENABLE_DILITHIUM


-- Build files have been written to: ~/aws-lc/build

This will also save some CI time by reducing the total number of builds in the posix tests by 1.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.74%. Comparing base (dc5fe84) to head (4b6efb3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2304   +/-   ##
=======================================
  Coverage   78.73%   78.74%           
=======================================
  Files         616      616           
  Lines      107584   107585    +1     
  Branches    15286    15287    +1     
=======================================
+ Hits        84709    84713    +4     
+ Misses      22220    22216    -4     
- Partials      655      656    +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.