[llvm-config] Remove --src-root option

Remove the `--src-root` option from the deprecated llvm-config tool.
None of the llvm-project projects use this option anymore. The value
was only meaningful for in-tree use and usually became no longer correct
once LLVM was installed -- either because it was built in a temporary
directory, or installed from a binary package and built on a different
system entirely.  Therefore, third-party tools could not have been
relying on it anyway.

The LLVM_SRC_ROOT #define is left intact, as it is used to compute
includedir when llvm-config is used in-source.

Differential Revision: https://reviews.llvm.org/D137144