[gn build] Port 9ec75a5

Author: aeubanks

llvm/utils/gn/secondary/llvm/lib/Target/Mips/MCTargetDesc/BUILD.gn

@@ -67,7 +67,6 @@ static_library("MCTargetDesc") {
     "MipsInstPrinter.cpp",
     "MipsMCAsmInfo.cpp",
     "MipsMCCodeEmitter.cpp",
-    "MipsMCExpr.cpp",
     "MipsMCTargetDesc.cpp",
     "MipsNaClELFStreamer.cpp",
     "MipsOptionRecord.cpp",