Remove docstring relicts of findlast/findprev with a regex

[laborg]

Discussion: https://discourse.julialang.org/t/findlast-pattern-regex-string-string-missing/15562/8

[KristofferC]

Seems like this docstring came from 93eef2d which also added

@deprecate rsearch(s::AbstractString, r::Regex, idx::Integer) findprev(r, s, idx)
@deprecate rsearch(s::AbstractString, r::Regex) findlast(r, s)

but where these rsearch methods ever defined? @nalimilan

[nalimilan]

I guess I introduced these by mistake when trying to make docstrings more consistent between findfirst/findnext and findlast/findprev.