Remove ERROR_DUPLICATE_FILE error from strings_english.txt

This was originally removed by 08a3d4f

A recent commit by grangeway d679a60
reintroduced this string, but since the constant does not exist it
generates a system notice which prevents MantisBT from functioning
properly (impossible to login).

lang/strings_english.txt

@@ -194,7 +194,6 @@ $MANTIS_ERROR[ERROR_REPORT] = 'There was an error in your report.';
 $MANTIS_ERROR[ERROR_NO_FILE_SPECIFIED] = 'No file specified.';
 $MANTIS_ERROR[ERROR_FILE_DISALLOWED] = 'The file type is disallowed.';
 $MANTIS_ERROR[ERROR_NO_DIRECTORY] = 'The directory does not exist. Please check the project settings.';
-$MANTIS_ERROR[ERROR_DUPLICATE_FILE] = 'This is a duplicate file. Please delete the file first.';
 $MANTIS_ERROR[ERROR_DUPLICATE_PROJECT] = 'A project with that name already exists.';
 $MANTIS_ERROR[ERROR_EMPTY_FIELD] = 'A necessary field "%1$s" was empty. Please recheck your inputs.';
 $MANTIS_ERROR[ERROR_PROTECTED_ACCOUNT] = 'This account is protected. You are not allowed to access this until the account protection is lifted.';
@@ -1641,4 +1640,4 @@ $s_month_august = 'August';
 $s_month_september = 'September';
 $s_month_october = 'October';
 $s_month_november = 'November';
-$s_month_december = 'December';
+$s_month_december = 'December';