generics: remove calls to `reflectutil.MustBeOfKind` in checker providers methods

[GregoryAlbouy]

Description

reflectutil.MustBeOfKind is used to ensure the underlying type of an interface{} parameter is an expected kind.

MapCheckerProvider and SliceCheckerProvider now being correctly typed with generics, reflectutil.MustBeOfKind calls in their methods became superfluous and should be removed.

Todo

• Remove calls to reflectutil.MustBeOfKind in MapCheckerProvider methods
• Remove calls to reflectutil.MustBeOfKind in SliceCheckerProvider methods