[LLVMSupport]: Remove a severely outdated README.

The LLVM Support library implementation has resided in
//llvm/lib/Support for a significant amount of time now,
with documentation having been updated with all references
to the "System library" being replaced with "Support library".

Since this file mirrors already existing documentation available
for Support library, includes dead links to documentation and
still refers to it as "System library", having it there is
confusing and updating it has very little point as it duplicates
information in documentation, except documentation is a lot more
up to date while this file has not been maintained.

Up to date documentation concerning this can be found here:
  http://llvm.org/docs/SupportLibrary.html

llvm-svn: 354209