intel: Remove redundant GL_ARB_window_pos enable

This extension is enabled by default in _mesa_init_extensions, so
drivers don't need to enable it again.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/intel/intel_extensions.c

    ctx->Extensions.ARB_vertex_array_object = true;
    ctx->Extensions.ARB_vertex_program = true;
    ctx->Extensions.ARB_vertex_shader = true;
-   ctx->Extensions.ARB_window_pos = true;
    ctx->Extensions.EXT_blend_color = true;
    ctx->Extensions.EXT_blend_equation_separate = true;
    ctx->Extensions.EXT_blend_func_separate = true;