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/screen

@@ -1,5 +1,5 @@
 # GNU Screen stuff
-if ( command -v screen &> /dev/null )
+if command -v screen &> /dev/null
 then
   if [[ ${TERM} != screen* ]]
   then