[gn] Remove unneeded MC dep from llvm-tblgen

Tablegen does not have link time dependencies on MC. Having llvm-tblgen
depend on it causes it to be rebuilt in the gn build every time somebody
touches any cpp file in llvm/lib/MC* or llvm/lib/DebugInfo/Codeview*.
Touching tablegen invalidates most of the rest of the build, and
re-running it takes a while. This is is annoying for me when swapping
between branches that touch CodeView logic.

This dep was added to LLVMBuild.txt back in 2018, and presumably it was
carried over into the gn build.

Differential Revision: https://reviews.llvm.org/D87553