Remove must_use from ALLOWED_ATTRIBUTES

This is a fairly common attribute on methods, but is not something you need to know when reading the method docs - the purpose of the attribute is for the compiler to tell you about it if you forget to use a value.

Removing reclaims some valuable space in the summary of methods, particularly when the attribute has a long string value.

As discussed in #84309. Partially addresses #81482.

r? ```@Manishearth```