pgsql: 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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2c75f8a612b207c7d36e5dc73317dc9ab6fb29d4

Modified Files
--------------
configure | 13 -------------
configure.ac | 3 +--
src/include/pg_config.h.in | 3 ---
src/tools/msvc/Solution.pm | 1 -
4 files changed, 1 insertion(+), 19 deletions(-)