Removing unnecessary psalm suppress docblock

[Ilyes512]

This is more of a FYI...

I am running psalm on my own project with the --find-unused-psalm-suppress flag and even though I told psalm to ignore the vendor dir it still showed me this error:

ERROR: UnusedPsalmSuppress - vendor/myclabs/php-enum/src/Enum.php:72:29 - This suppression is never used (see https://psalm.dev/207)
/** @psalm-suppress ImplicitToStringCast assertValidValueReturningKey returns always a string but psalm has currently an issue here */

So in other words I think this line can be removed: https://github.com/myclabs/php-enum/blob/master/src/Enum.php#L72