`MacOS::Xcode.without_clt? && MacOS.sdk_path.nil?` should never be true.

In its earliest form, this would raise a bare RuntimeError in an effort
to have the bug reported. Later, it was changed to silently disable
superenv. But we don't want to do that. If there's a bug, or the user's
system is misconfigured, we want to know, so that we can fix the bug, or
the user can fix their system. So let's remove the condition.

Closes Homebrew/homebrew#28588.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Closes Homebrew/homebrew#28550.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

These days Homebrew refuses to do anything if the xcode-select path is
busted, so this workaround is unnecessary.