-march not required if -mcpu is set

GCC_ARM throws warning if both architecture and core are set (though
correct). If CPU option is set correctly, architecture is set by compiler
itself.

tools/toolchains/gcc.py

@@ -86,11 +86,6 @@ class GCC(mbedToolchain):
             self.cpu.append("-mfloat-abi=hard")
             self.cpu.append("-mno-unaligned-access")
 
-        if target.core.startswith("Cortex-M23"):
-            self.cpu.append("-march=armv8-m.base")
-        elif target.core.startswith("Cortex-M33"):
-            self.cpu.append("-march=armv8-m.main")
-
         if target.core == "Cortex-M23" or target.core == "Cortex-M33":
             self.cpu.append("-mcmse")