[compiler-rt] Drop COMPILER_RT_BUILD_CRT workaround (#78331)

This variable was explicitly removed from the cache to ease transition
from existing build directories but that breaks passing
COMPILER_RT_BUILD_CRT=OFF on the command line. I was surprised to see
the CRT builds being run for my builtins-only build config (I noticed
because one of the tests was failing despite having `REQUIRES: crt`).

If I pass `-DCOMPILER_RT_BUILD_CRT=OFF` to cmake and add some prints
around the `unset` statement it shows the following:
```
-- before unset(): COMPILER_RT_BUILD_CRT=OFF
-- after unset: COMPILER_RT_BUILD_CRT=
-- after cmake_dependent_option COMPILER_RT_BUILD_CRT=ON
```

Drop this temporary workaround now that over 6 months have passed.

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