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

configure

@@ -13582,15 +13582,6 @@ fi
 fi
 
 if test "$with_lz4" = yes; then
-  for ac_header in lz4/lz4.h
-do :
-  ac_fn_c_check_header_mongrel "$LINENO" "lz4/lz4.h" "ac_cv_header_lz4_lz4_h" "$ac_includes_default"
-if test "x$ac_cv_header_lz4_lz4_h" = xyes; then :
-  cat >>confdefs.h <<_ACEOF
-#define HAVE_LZ4_LZ4_H 1
-_ACEOF
-
-else
   for ac_header in lz4.h
 do :
   ac_fn_c_check_header_mongrel "$LINENO" "lz4.h" "ac_cv_header_lz4_h" "$ac_includes_default"
@@ -13607,10 +13598,6 @@ done
 
 fi
 
-done
-
-fi
-
 if test "$with_gssapi" = yes ; then
   for ac_header in gssapi/gssapi.h
 do :

configure.ac

@@ -1440,8 +1440,7 @@ Use --without-zlib to disable zlib support.])])
 fi
 
 if test "$with_lz4" = yes; then
-  AC_CHECK_HEADERS(lz4/lz4.h, [],
-       [AC_CHECK_HEADERS(lz4.h, [], [AC_MSG_ERROR([lz4.h header file is required for LZ4])])])
+  AC_CHECK_HEADERS(lz4.h, [], [AC_MSG_ERROR([lz4.h header file is required for LZ4])])
 fi
 
 if test "$with_gssapi" = yes ; then

src/include/pg_config.h.in

@@ -364,9 +364,6 @@
 /* Define to 1 if you have the <lz4.h> header file. */
 #undef HAVE_LZ4_H
 
-/* Define to 1 if you have the <lz4/lz4.h> header file. */
-#undef HAVE_LZ4_LZ4_H
-
 /* Define to 1 if you have the <mbarrier.h> header file. */
 #undef HAVE_MBARRIER_H
 

src/tools/msvc/Solution.pm

@@ -313,7 +313,6 @@ sub GenerateFiles
 		HAVE_LONG_INT_64            => undef,
 		HAVE_LONG_LONG_INT_64       => 1,
 		HAVE_LZ4_H                  => undef,
-		HAVE_LZ4_LZ4_H              => undef,
 		HAVE_MBARRIER_H             => undef,
 		HAVE_MBSTOWCS_L             => 1,
 		HAVE_MEMORY_H               => 1,