meson: remove dependency antipattern

meson: remove dependency antipattern

`dep_valgrind != []` now (0.45) produces a warning that is quite explicit:
  WARNING: Trying to compare values of different types (DependencyHolder, list) using !=.
  The result of this is undefined and will become a hard error in a future Meson release.

`dep_valgrind = []` used to be the recommended way to deal with
non-existant dependency, but these don't work with `.found()`, so now
the recommended way is to declare a impossible dependency, which
null_dep does for us in Mesa.

In short, we don't need and shouldn't check for `!= []` anywhere anymore.

Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>