perf: remove unnecessary subshells in command -v checks

Replace 'if (command -v x &>/dev/null); then' with
'if command -v x &>/dev/null; then' across completion.d, rc.d and
profile.d. Parentheses forced a subshell fork for every 'if' check,
which is unnecessary overhead since command -v runs fine in the
current shell.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

rc.d/tmux

@@ -1,5 +1,5 @@
 # GNU Screen stuff
-if (command -v tmux &>/dev/null); then
+if command -v tmux &>/dev/null; then
   if [ "$(pgrep -u "${UID}" "tmux" | wc -l)" -gt 0 ]; then
     if [[ ${TERM} != screen* ]]; then
       nb_sessions=$(tmux list-session | wc -l)