[llvm] Remove unused file MaximumSpanningTree.h

The last use of this file was removed in late 2013 in ea564946.
The last use was in PathProfiling.cpp, which had an overview comment
of the overall approach.

Similar functionality lives in the slight more cryptically named
CFGMST.h in this same directory. A similar overview comment is
in PGOInstrumentation.cpp.

No behavior change.