[lldb][NFC] Remove unused custom reimplementation of realpath for Windows

No one is calling this function it seems and according to
https://bugs.llvm.org/show_bug.cgi?id=47088 this can leak memory, so let's just
remove it:

Quote from the bug report:
> Before return on line 146, the memory allocated on line 130 is not freed.

Reviewed By: amccarth

Differential Revision: https://reviews.llvm.org/D85633