grep-jump and make-jump[*] support Windows-style paths like C:\path.
However grep-next-match and make-next-error don't, which suggests
that no one uses this feature. IIRC Cygwin mounts Windows drives in
proper Unix paths like /mnt/drive_c.
Let's remove it for simplicity and consistency.
This reverts commit 6c47b204e (Support windows style path in grep
output, 2014-11-11).
[*]: Though make-jump recently regressed in 8e5ca3f21 (rc/make.kak
introduce a new option to be back compatible, 2023-10-27) by failing
to capture the "C:" prefix.
20d353936e