Revert "msvc: enable /std:c11 flag"

This reverts commit b5edb514d7, which
was not supposed to be cherry-picked to the stable branch.

It causes projects that were previously working and had c_std=gnu99 in
default_options to error out with:

```
meson.build:1:0: ERROR: Value "gnu99" for combo option "C language
standard to use" is not one of the choices. Possible choices are:
"none", "c89", "c99", "c11".
```

mesonbuild/compilers/c.py

@@ -353,26 +353,6 @@ class VisualStudioCCompiler(MSVCCompiler, VisualStudioLikeCCompilerMixin, CCompi
                            info, exe_wrap, **kwargs)
         MSVCCompiler.__init__(self, target)
 
-    def get_options(self):
-        opts = super().get_options()
-        c_stds = ['none', 'c89', 'c99', 'c11']
-        opts.update({
-            'std': coredata.UserComboOption(
-                'C language standard to use',
-                c_stds,
-                'none',
-            ),
-        })
-        return opts
-
-    def get_option_compile_args(self, options):
-        args = []
-        std = options['std']
-        # As of MVSC 16.7, /std:c11 is the only valid C standard option.
-        if std.value in {'c11'}:
-            args.append('/std:' + std.value)
-        return args
-
 
 class ClangClCCompiler(ClangClCompiler, VisualStudioLikeCCompilerMixin, CCompiler):
     def __init__(self, exelist, version, for_machine: MachineChoice,