Remove definition of __WASM_EXCEPTIONS__. NFC (#25295)

This macros is automatically defined clang in the pre-processor when
wasm exceptions are enabled.

See
https://github.com/llvm/llvm-project/blob/82acc31fd4b26723b61dae76da120c0c649d0b97/clang/lib/Frontend/InitPreprocessor.cpp#L1034-L1035

Author: sbc100

tools/system_libs.py

@@ -1641,8 +1641,6 @@ def get_cflags(self):
       # The code used to interpret exceptions during terminate
       # is not compatible with emscripten exceptions.
       cflags.append('-DLIBCXXABI_SILENT_TERMINATE')
-    elif self.eh_mode == Exceptions.WASM_LEGACY:
-      cflags.append('-D__WASM_EXCEPTIONS__')
     return cflags
 
   def get_files(self):
@@ -1721,12 +1719,6 @@ class libcxx(ExceptionLibrary, MTLibrary, DebugLibrary):
     'tzdb_list.cpp',
   ]
 
-  def get_cflags(self):
-    cflags = super().get_cflags()
-    if self.eh_mode in (Exceptions.WASM_LEGACY, Exceptions.WASM):
-      cflags.append('-D__WASM_EXCEPTIONS__')
-    return cflags
-
 
 class libunwind(ExceptionLibrary, MTLibrary):
   name = 'libunwind'
@@ -1757,8 +1749,6 @@ def get_cflags(self):
       cflags.append('-D_LIBUNWIND_HAS_NO_EXCEPTIONS')
     elif self.eh_mode == Exceptions.EMSCRIPTEN:
       cflags.append('-D__EMSCRIPTEN_EXCEPTIONS__')
-    elif self.eh_mode in (Exceptions.WASM_LEGACY, Exceptions.WASM):
-      cflags.append('-D__WASM_EXCEPTIONS__')
     return cflags