BaseTools/Conf: Don't support upper case nasm extensions

BaseTools/Conf: Don't support upper case nasm extensions

For *.asm and *.s, there have been cases of *.Asm and *.S files, but
since the nasm extensions are new, we don't need to support the upper
case extensions.

In other words, remove .Nasm and .NASM.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17544 6f19259b-4bc3-4df7-8a09-765794883524