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>

completion.d/stern

@@ -1,4 +1,4 @@
-if (command -v stern &> /dev/null)
+if command -v stern &> /dev/null
 then
   source <(stern --completion bash)
 fi