[llvm-mca] Remove README.txt
[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.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@338386 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm