Remove a use of n_spaces

While removing printfi_filtered, I found a spot that used n_spaces
where the now-ordinary "%*s" approach would do.  This patch makes this
change.

Tested on x86-64 Fedora 32.

gdb/ChangeLog
2020-12-17  Tom Tromey  <tromey@adacore.com>

	* printcmd.c (print_variable_and_value): Don't use n_spaces.