Platform: Remove forced SST_RAM_FS=ON on Musca-A

Removes override for SST_RAM_FS=ON for Musca-A, as the board is able to
run with the RAM FS off.

Change-Id: I3b60b0be2855a1a7e98c06c5ed69a220bd157b4b
Signed-off-by: Jamie Fox <jamie.fox@arm.com>