[WIP] Remove BACKSLASH_IN_FILENAME

According to @Hinidu [here] (https://github.com/neovim/neovim/pull/447#discussion_r11269484):

> It ignores slash/backslash differences only if BACKSLASH_IN_FILENAME defined. In vim it is defined only in src/os_os2_cfg.h and src/os_dos.h. So it is not needed in modern neovim.

This will simplify much of the code in `path.c` and in other places.