fix gnome-shell

• src/meson.build does not contain %%PYTHON_CMD%%, so that REINPLACE is redundant
• toplevel meson.build searches for python3, which is a BINARY_ALIAS and thus hardcodes the path to that BINARY_ALIAS into some files, which is absolutely not helpful. instead, point meson to the fully versioned python interpreter PYTHON_VERSION
• the bash completions are gone, having them in plist prevents packaging

Approved by: just-fix-it-before-next-quarterly