Removes parameter 'P' from Shader::from_vertex_string and Shader::from_fragment_string

[LiquidityC]

Fixes: #300

[17cupsofcoffee]

Wow, this has been broken for two whole years and nobody's noticed 🤦 I think I created these methods by copying the Path versions and forgot to remove the generics. Good spot!

I'm honestly surprised Rust doesn't mark this as an error/warning like it does for unused generics on a struct.

This is technically a breaking change (as you could work around the issue by turbofishing the method with any random type), but I don't think anyone would complain if this was fixed in a patch release, so I'll merge this into main rather than 0.7.

[17cupsofcoffee]

Thank you :)