Revert "unpack-trees: improve performance of next_cache_entry"

Revert "unpack-trees: improve performance of next_cache_entry"

This reverts commit f2a454e0a5 (unpack-trees: improve performance of
next_cache_entry, 2021-11-29).

The "hint" value was originally needed to improve performance in 'git reset
-- <pathspec>' caused by 'cache_bottom' lagging behind its correct value
when using a sparse index. The 'cache_bottom' tracking has since been
corrected, removing the need for an additional "pseudo-cache_bottom"
tracking variable.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>