build.py: Simplify validate_sources()

There is no need to go through all sources again, we already did that to
populate self.compilers. When cs or java compilers are in the list, then
there must be only one compiler.

The code was also not considering generate sources any way.

mesonbuild/build.py

@@ -971,21 +971,9 @@ class BuildTarget(Target):
         return missing_languages
 
     def validate_sources(self):
-        if not self.sources:
-            return
-        for lang in ('cs', 'java'):
-            if lang in self.compilers:
-                check_sources = list(self.sources)
-                compiler = self.compilers[lang]
-                if not self.can_compile_remove_sources(compiler, check_sources):
-                    raise InvalidArguments(f'No {lang} sources found in target {self.name!r}')
-                if check_sources:
-                    m = '{0} targets can only contain {0} files:\n'.format(lang.capitalize())
-                    m += '\n'.join([repr(c) for c in check_sources])
-                    raise InvalidArguments(m)
-                # CSharp and Java targets can't contain any other file types
-                assert len(self.compilers) == 1
-                return
+        if len(self.compilers) > 1 and any(lang in self.compilers for lang in {'cs', 'java'}):
+            langs = ', '.join(self.compilers.keys())
+            raise InvalidArguments(f'Cannot mix those languages into a target: {langs}')
 
     def process_link_depends(self, sources, environment):
         """Process the link_depends keyword argument.