build: Retire LLVM_COV src.conf knob

[emaste]

Summary

I introduced it in commit 1b49115a40ad ("Promote llvm-cov to a
standalone option").  llvm-cov was previously enabled as part of the
CLANG_EXTRAS option.  I made it a standalone, default-enabled option for
parity with the tools provided by the GCC-based toolchain.

We no longer provide an in-tree GCC toolchain.  Now, just build llvm-cov
along with Clang to simplify build infrastructure.

[dim]

LGTM. Good riddance. :)