Remove unused searchhelp-url and searchmenu-help

searchhelp-url was introduced around r43595 to improve
searchmenu-help (r43376). Both were part of a Special:Search redesign
that never really happened. Commented out in r50207, and removed
in r51228

See also Ic0469c6c51a4d54ee214347fc416615c2f6cb1f0

Change-Id: I82259bd836ba8c4a3042cb81c99165c312ca04cc