build: remove some no-longer-needed var unexporting from configure

configure.ac

@@ -1959,14 +1959,6 @@ LIBS_TEMP="$LIBS"
 unset LIBS
 LIBS="$LIBS_TEMP"
 
-PKGCONFIG_PATH_TEMP="$PKG_CONFIG_PATH"
-unset PKG_CONFIG_PATH
-PKG_CONFIG_PATH="$PKGCONFIG_PATH_TEMP"
-
-PKGCONFIG_LIBDIR_TEMP="$PKG_CONFIG_LIBDIR"
-unset PKG_CONFIG_LIBDIR
-PKG_CONFIG_LIBDIR="$PKGCONFIG_LIBDIR_TEMP"
-
 ac_configure_args="${ac_configure_args} --disable-shared --with-pic --enable-benchmark=no --enable-module-recovery --enable-module-schnorrsig"
 AC_CONFIG_SUBDIRS([src/secp256k1])