Delete multi-byte related functions from PAL (#41611)

Delete multi-byte related functions from PAL (#41611)

* Delete multi-byte related functions from PAL

In PAL, delete:
* `IsDBCSLeadByte`; unimplemented.
* `IsDBCSLeadByteEx`; no-op.
* `GetCPInfo`; has hardcoded length of 4, and the only usage checks for
  `length == 1`.
* `_mbsinc`; has one usage in ilasm, which is simplified for Unix
  (after the removal of above).
* `_mbsninc`; unused.
* `_mbsdec`; usage is inlined as `p--` because `IsDBCSLeadByteEx` is a
  no-op.
* `GetCaseInsensitiveValueA`, handle ANSI cases as Unicode.
* `IsTextUnicode`, single usage in ilasm is inlined.

* Delete _MBCS, _ismbblead and simplifications

* Delete trailing whitespaces from PR file changed