Remove unnecessary "#include <specularmap_fragment>" from meshphysical_frag.glsl

[takahirox]

This PR removes unnecessary #include <specularmap_fragment> from meshphysical_frag.glsl which is shader for MeshStandardMaterial and MeshPhysicalMaterial.

If I'm right, meshphysical_frag.glsl doesn't need to include specularmap_fragment.

specularmap_fragment.glsl is a shader chunk for specularStrength.
specularStrength is used only in envmap_fragment.glsl and lights_phong_[pars_]fragment.glsl but
meshphysical_frag.glsl doesn't include them.

[mrdoob]

@WestLangley looks good?

[WestLangley]

Yes, that can be removed.

#include <specularmap_pars_fragment> in the vertex shader can also be removed, I expect.

Please confirm. Thanks.

[takahirox]

Yup, seems like we also can remove it.
Confirmed and updated.

[mrdoob]

Thanks!