change default uninstall/clean keybinding to prefix + alt +u

scripts/variables.sh

@@ -5,4 +5,4 @@ update_key_option="@tpm-update"
 default_update_key="U"
 
 clean_key_option="@tpm-clean"
-default_clean_key="Y"
+default_clean_key="M-u"

tests/expect_successful_clean_plugins

@@ -9,8 +9,8 @@ spawn tmux
 # not work properly.
 sleep 1
 
-# this is tmux prefix + U
-send "Y"
+# this is tmux prefix + alt + u
+send "u"
 
 set timeout 5