maint: remove unnecessary configure checks for libdl and dlopen

Since commit bff0e5412 (build: sort: explicitly tag libcrypto
dependency, 2026-09-06), which added a dependency on Gnulib's sd-dlopen
module, this check has been unnecessary. This can be seen by the
separate variables in Makefile.in, which have the same purpose.

    $ grep -E '^LIB_?DL' Makefile.in
    LIBDL = @LIBDL@
    LIB_DL = @LIB_DL@

* src/local.mk (src_sort_LDADD): Use $(LIBDL) instead of $(LIB_DL).
* configure.ac: Likewise. Don't check for dlopen and libdl, which Gnulib
has already done for us.

configure.ac

@@ -304,15 +304,6 @@ AC_CHECK_FUNCS([sigsuspend],
 
 gl_WINSIZE_IN_PTEM
 
-# Find the library for dynamic loading of shared libraries.
-AC_SEARCH_LIBS([dlopen], [dl])
-AS_CASE([$ac_cv_search_dlopen],
-  [no | 'none required'],
-    [LIB_DL=],
-  [*],
-    [LIB_DL="$ac_cv_search_dlopen"])
-AC_SUBST([LIB_DL])
-
 # Should 'sort' link libcrypto dynamically?
 AS_CASE([$LIB_CRYPTO],
   [-lcrypto],
@@ -322,7 +313,7 @@ AS_CASE([$LIB_CRYPTO],
        [utils_cv_libcrypto_soname],
        [utils_cv_libcrypto_soname=no
         saved_LIBS=$LIBS
-        LIBS="$LIBS $LIB_DL $LIB_CRYPTO"
+        LIBS="$LIBS $LIBDL $LIB_CRYPTO"
         AC_LINK_IFELSE(
           [AC_LANG_PROGRAM(
              [[#include <dlfcn.h>

src/local.mk

@@ -323,7 +323,7 @@ src_printf_LDADD += $(LIBICONV)
 
 # for libcrypto hash routines
 src_md5sum_LDADD += $(LIB_CRYPTO)
-src_sort_LDADD += $(LIB_DL) $(LIB_CRYPTO)
+src_sort_LDADD += $(LIBDL) $(LIB_CRYPTO)
 src_sha1sum_LDADD += $(LIB_CRYPTO)
 src_sha224sum_LDADD += $(LIB_CRYPTO)
 src_sha256sum_LDADD += $(LIB_CRYPTO)