Remove FILEDosToUnixPathA conversion (#78995)

Remove FILEDosToUnixPathA conversion (#78995)

* Remove FILEDosToUnixPathA conversion

All file paths passed to the coreclr PAL are processed by this function
to convert backslash characters to forward ones. This is causing problems
when users have directories with names containing backslashes. Such
problems were recently reported by people.

This change removes the `FILEDosToUnixPathA` and fixes one PAL test and
one coreclr test that was passing backslash separated paths.

* Remove checks for `\` on Unix and some conversions

* Update runtime to use platform specific directory separator

* Revert changes at few places and remove some code for Unix

* Fix Bundle_Extraction_To_Relative_Path_Succeeds host test

Exclude the `foo\\bar` case for non-Windows.