Remove pointless install_umask validation check for None

This option can never have a value of None. There are only two sources
of values at all:

- the class instance initializer when defining BUILTIN_CORE_OPTIONS
- user-provided command-line or machine file values etc. which can only
  be meson types

We know we don't construct the Option instance with None, and users
cannot pass a None anywhere since that's not a meson type. The only
reason this was ever checked for was as an artifact during the initial
implementation of the option in commit 8651d55c6a.

At the time, a review comment was made that `-Dinstall_umask=none` was a
bad UX and "preserve" should be used instead. Before that, this option
type accepted `None` (in the BUILTIN_CORE_OPTIONS initializer) and
`'none'` (provided by users) which is odd and should have consistently
been the latter. Then inside set_value, it checked for the magic
initializer value and converted it to the real value.

After review comments and a force-push, the patch ended up using `None`
in the initializer, and `'preserve'` everywhere else, and still handling
both in set_value and converting both to a proper string.

In the very next commit in the patch series, the initializer was
migrated to use an actual umask of 022, and now `None` was entirely
impossible to get anywhere at all. But the wart of checking for it was
never removed. Remove it at long last.

mesonbuild/coredata.py

@@ -191,7 +191,7 @@ class UserUmaskOption(UserIntegerOption, UserOption[T.Union[str, OctalInt]]):
         return format(self.value, '04o')
 
     def validate_value(self, value: T.Any) -> T.Union[str, OctalInt]:
-        if value is None or value == 'preserve':
+        if value == 'preserve':
             return 'preserve'
         return OctalInt(super().validate_value(value))