Reduce once MB_ENABLED checking

The checking is done in the compat file system/core/compat/mbstring.php
diff --git a/system/libraries/Form_validation.php b/system/libraries/Form_validation.php
index f161b40..9d16602 100644
--- a/system/libraries/Form_validation.php
+++ b/system/libraries/Form_validation.php
@@ -1151,9 +1151,7 @@
 			return FALSE;
 		}
 
-		return (MB_ENABLED === TRUE)
-			? ($val <= mb_strlen($str))
-			: ($val <= strlen($str));
+		return ($val <= mb_strlen($str));
 	}
 
 	// --------------------------------------------------------------------
@@ -1172,9 +1170,7 @@
 			return FALSE;
 		}
 
-		return (MB_ENABLED === TRUE)
-			? ($val >= mb_strlen($str))
-			: ($val >= strlen($str));
+		return ($val >= mb_strlen($str));
 	}
 
 	// --------------------------------------------------------------------
@@ -1193,9 +1189,7 @@
 			return FALSE;
 		}
 
-		return (MB_ENABLED === TRUE)
-			? (mb_strlen($str) === (int) $val)
-			: (strlen($str) === (int) $val);
+		return (mb_strlen($str) === (int) $val);
 	}
 
 	// --------------------------------------------------------------------