[llvm-mca] Remove README.txt

A detailed description of the tool has been recently added by Matt to
CommandGuide/llvm-mca.rst. File README.txt is now redundant and can be removed;
all the relevant user-guide information has been improved and then moved to
llvm-mca.rst.

In future, we should add another .rst for the "llvm-mca developer manual" to
provide infromation about:
 - llvm-mca internals.
 - How to add custom stages to the simulated pipeline.
 - How to provide extra processor info in the scheduling model to improve the
   analysis performed by llvm-mca.

llvm-svn: 338386