build: remove unneeded var exporting

Author: fanquake

configure.ac

@@ -2004,14 +2004,6 @@ CPPFLAGS_TEMP="$CPPFLAGS"
 unset CPPFLAGS
 CPPFLAGS="$CPPFLAGS_TEMP"
 
-LDFLAGS_TEMP="$LDFLAGS"
-unset LDFLAGS
-LDFLAGS="$LDFLAGS_TEMP"
-
-LIBS_TEMP="$LIBS"
-unset LIBS
-LIBS="$LIBS_TEMP"
-
 ac_configure_args="${ac_configure_args} --disable-shared --with-pic --enable-benchmark=no --enable-module-recovery --disable-module-ecdh"
 AC_CONFIG_SUBDIRS([src/secp256k1])