vapigen: Don't override VALAC and use the available one like the rest
vapigen: Don't override VALAC and use the available one like the rest

There is no reason to special case vapigen while the rest of the source
is built with the available valac version. For a proper build bootstrapping
is required anyway.

For internal testing of the just-built valac the testsuite should suffice.

GNOME/vala,GNOME/vala,rvu95/vala,rvu95/vala,spidermonkdust/vala,spidermonkdust/vala,flobrosch/vala-fixes,frida/vala,flobrosch/vala-fixes,frida/vala,rvu95/vala,GNOME/vala,GNOME/vala,spidermonkdust/vala,frida/vala,flobrosch/vala-fixes,frida/vala,frida/vala