etnaviv: avoid re-defining prog_python

It's already defined to the exact same thing in the root `meson.build`

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28801>

src/etnaviv/hwdb/meson.build

@@ -3,7 +3,6 @@
 # SPDX-License-Identifier: MIT
 #
 
-prog_python = import('python').find_installation('python3')
 has_pycparser = run_command(
   prog_python, '-c',
   '''