Remove useless configure probe for <lz4/lz4.h>.

Remove useless configure probe for <lz4/lz4.h>.

This seems to have been just copied-and-pasted from some other
header checks.  But our C code is entirely unprepared to support
such a header name, so it's only wasting cycles to look for it.
If we did need to support it, some #ifdefs would be required.

(A quick trawl at codesearch.debian.net finds some packages that
reference lz4/lz4.h; but they use *only* that spelling, and
appear to be intending to reference their own copy rather than
a system-level installation of liblz4.  There's no evidence of
freestanding installations that require this spelling.)

Discussion: https://postgr.es/m/457962.1616362509@sss.pgh.pa.us