shaders: remove the .so plugin loader

The static musl release builds cannot dlopen -- musl compiles it out
-- so loadable shader plugins were a feature most shipped binaries
could not use. Nothing we ship used it either: every shader in
contrib/shaders is an expression. Gone: the dlopen/glob loader in
shader.c, shader_dir in the config, the plugin ABI table and entry
macro, the Windows dl*/glob shims that existed only for it, the
contrib/shader-plugin skeleton, its tests, and every doc mention.
shader_abi.h stays as the shader model (cell_t, shade_ctx_t,
shader_t) with a note about what it used to be.

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