SourceLevelDebugPkg/DebugAgentCommon: Remove .S files

.nasm file has been added for X86 arch. .S assembly code
is not required any more.
https://bugzilla.tianocore.org/show_bug.cgi?id=1594



Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>