remotefilelog: remove strkwargs()
The previous commit activated previously unused code paths on
Python 3 and revealed that we were calling strkwargs() on a dict
that already had str keys. The only caller of _forceprefetch()
is _adjustlinknode() a few lines up and the static commonlogkwargs
dict features str keys.
Differential Revision: https://phab.mercurial-scm.org/D5958