LibGfx/PNM: Remove two fixmes

bab2113ec1f made read_whitespace() return ErrorOr, which makes this
easy to do.

(7cafd7d177f, which added the fixmes, landed slightly after bab2113ec1f,
so not quite sure why it wasn't like this immediately. Maybe commit
order got changed during review; both commits were in #17831.)

No behavior change.

Userland/Libraries/LibGfx/ImageFormats/PortableImageLoaderCommon.h

@@ -116,12 +116,12 @@ static ErrorOr<void> read_whitespace(TContext& context)
         auto const byte = byte_or_error.value();
 
         if (byte == '#') {
-            stream.seek(-1, SeekMode::FromCurrentPosition).release_value_but_fixme_should_propagate_errors();
+            TRY(stream.seek(-1, SeekMode::FromCurrentPosition));
             TRY(read_comment(context));
             continue;
         }
         if (byte != ' ' && byte != '\t' && byte != '\n' && byte != '\r') {
-            stream.seek(-1, SeekMode::FromCurrentPosition).release_value_but_fixme_should_propagate_errors();
+            TRY(stream.seek(-1, SeekMode::FromCurrentPosition));
             if (is_first_char)
                 return Error::from_string_literal("Can't read whitespace from stream");
             break;