mesa: do not leak ctx->Shader.ReferencedProgram references

When glUseProgram is used, references to the included shaders are
added in ctx->Shader.ReferencedProgram. But those references are not
decreased when the shader data is deallocated. Thus, those shaders
are leaked.

Explicitely remove the pending references to these shaders.

Fixes: e6506b3c ("mesa: retain gl_shader_programs after glDeleteProgram if they are in use")
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
(cherry picked from commit 6c61c31d)