Don't use shebang for bash completion file

Bash completions are meant to be sourced, not executed, so remove the shebang line.

Debian lintian complains about it:

W: xfconf: bash-completion-with-hashbang usr/share/bash-completion/completions/xfconf-query
N:
W: bash-completion-with-hashbang
N:
N:   This file starts with the #! sequence that marks interpreted scripts,
N:   but it is a bash completion script that is merely intended to be
N:   sourced.
N:   
N:   Please remove the line with hashbang, including any designated
N:   interpreter.
N:   
N:   Refer to
N:   https://salsa.debian.org/lintian/lintian/-/merge_requests/292#note_139494
N:   for details.
N:   
N:   Severity: warning
N:   
N:   Check: scripts
N: