[chore] Fix a few Persian keyboard luacheck issues (#7253)

Cf. <https://github.com/koreader/koreader/pulls#issuecomment-774345230>.

frontend/ui/data/keyboardlayouts/fa_keyboard.lua

@@ -1,30 +1,19 @@
 local en_popup = require("ui/data/keyboardlayouts/keypopup/en_popup")
 local fa_popup = require("ui/data/keyboardlayouts/keypopup/fa_popup")
-local com = en_popup.com -- comma (,)
 local prd = en_popup.prd -- period (.)
 local _at = en_popup._at
-local _eq = en_popup._eq -- equals sign (=)
 local alef = fa_popup.alef
 local h_aa = fa_popup.h_aa -- This is Persian letter هـ / as in English "hello".
 local waw = fa_popup.waw
 local yaa = fa_popup.yaa
 local kaf = fa_popup.kaf
 local diacritics = fa_popup.diacritics
-local diacritic_fat_ha = fa_popup.diacritic_fat_ha
-local diacritic_damma = fa_popup.diacritic_damma
-local diacritic_kasra = fa_popup.diacritic_kasra
-local diacritic_sukoon = fa_popup.diacritic_sukoon
-local diacritic_shadda = fa_popup.diacritic_shadda
-local diacritic_tanween_fath = fa_popup.diacritic_tanween_fath
-local diacritic_tanween_damm = fa_popup.diacritic_tanween_damm
-local diacritic_tanween_kasr = fa_popup.diacritic_tanween_kasr
 local arabic_comma = fa_popup.arabic_comma
 
-
 return {
     min_layer = 1,
     max_layer = 4,
-    shiftmode_keys = {["1/2"] = true, ["2/2"] = true},                   
+    shiftmode_keys = {["1/2"] = true, ["2/2"] = true},
     symbolmode_keys = {["نشانه‌ها"] = true,["الفبا"]=true},      -- نشانه‌ها means "Symbol", الفبا means "letter" (traditionally "ABC" on QWERTY layouts)
     utf8mode_keys = {["🌐"] = true},                      -- The famous globe key for layout switching
     umlautmode_keys = {["Äéß"] = false},                  -- No need for this keyboard panel

frontend/ui/data/keyboardlayouts/keypopup/fa_popup.lua

@@ -7,7 +7,7 @@ return {
         east = "آ",
         west = "ﭐ",
     },
-    h_aa = { 
+    h_aa = {
         "ه",
        north = "ة",
     },