util: remove the copypasta unquote function
The _urlunquote function was added back in the day to improve startup
performance, but this version is a) not compatible with Python 3 and b) has
quadratic performance issues that Python core solved eons ago.
Moreover, the function moved from urllib to urlparse (cheaper to import) *and*
that module is already imported into pycompat. As a result, removing this
function improves perf now.
Before:
! wall 0.066773 comb 0.000000 user 0.000000 sys 0.000000 (best of 100)
After:
! wall 0.065990 comb 0.000000 user 0.000000 sys 0.000000 (best of 100)