Trac #30718: build/pkgs/pynac/dependencies: Remove giac

As noted in #21700, our `pynac` spkg-install script explicitly disables
use of giac. So that should not be a dependency.

Removing the dependency allows for greater parallelization of the build.

This is also a step toward allowing to build Sage with `./configure
--without-giac` (see #30383 - `configure --disable-SPKG` for standard
packages).

See also:
 - #29542 spkg-configure.m4 for pynac

URL: https://trac.sagemath.org/30718
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): John Palmieri

build/pkgs/pynac/dependencies

@@ -1,4 +1,4 @@
-$(PYTHON) $(MP_LIBRARY) flint singular giac | pkgconf
+$(PYTHON) $(MP_LIBRARY) flint singular | pkgconf
 
 ----------
 All lines of this file are ignored except the first.