BaseTools: Remove unused txt files

BaseTools: Remove unused txt files

externals.txt is to refer to the binary Win32. It is not used any longer.
BuildNotes.txt is to freeze BaseTools python. It is not used any longer.
BinaryFiles.txt is to list the file in Bin directory. But, Bin dir is empty.
building-gcc.txt and gcc is for mingw-gcc. It is not used any longer.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Liming Gao <liming.gao@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>