Get rid of useless/confusing subshell

Get rid of useless/confusing subshell

This function's style is a bit off:  it wraps the body in a subshell to
discard all stdout/err at once, but still uses return inside it.

1. A command list (using {}) would be enough here as it groups like a
   subshell but avoids spawning another shell;
2. discarding stdout/err at the end of an if block works the same
   (effecting both condition and body) and saves one level of indent;
3. return inside a subshell inside a function does NOT return from the
   function but merely exits the subshell;  this is easily misread.

Saving a fork and indent and improving readability boils down to this
(cvs diff -wU1):

	|@@ -3320,3 +3317,2 @@ check_unattendedupgrade() {
	| 	_d=${_d%% *}
	|-	(
	| 		if [[ -n $_d ]]; then
	|@@ -3331,5 +3327,5 @@ check_unattendedupgrade() {
	| 			rm -f /dev/{r,}$_d?
	|-		fi
	|+	fi >/dev/null 2>&1
	|+
	| 		return $_rc
	|-	) > /dev/null 2>&1
	| }

OK halex