Revert "Detect NulInCStr error earlier."

This reverts commit 6001c50.

Author: lnicola

crates/parser/src/lexed_str.rs

@@ -369,7 +369,6 @@ fn error_to_diagnostic_message(error: EscapeError, mode: Mode) -> &'static str {
             "non-ASCII character in byte string literal"
         }
         EscapeError::NonAsciiCharInByte => "non-ASCII character in raw byte string literal",
-        EscapeError::NulInCStr => "null character in C string literal",
         EscapeError::UnskippedWhitespaceWarning => "",
         EscapeError::MultipleSkippedLinesWarning => "",
     }

crates/syntax/src/validation.rs

@@ -106,9 +106,6 @@ fn rustc_unescape_error_to_string(err: unescape::EscapeError) -> (&'static str,
         EE::NonAsciiCharInByte  => {
             "Byte literals must not contain non-ASCII characters"
         }
-        EE::NulInCStr  => {
-            "C strings literals must not contain null characters"
-        }
         EE::UnskippedWhitespaceWarning => "Whitespace after this escape is not skipped",
         EE::MultipleSkippedLinesWarning => "Multiple lines are skipped by this escape",