graphics/mesa-dri: drop USE_LDCONFIG due to dlopen(3)

OpenGL drivers in mesa-dri lack headers and cannot be used by ld(1)
via -l<lib> due to missing "lib" prefix. libglvnd uses dlopen(3)
to find lib(EGL|GLX)_mesa.so in mesa-libs which uses dlopen(3)
to find the drivers under ${PREFIX}/lib/dri (or LIBGL_DRIVERS_PATH).

Vulkan drivers in mesa-dri lack headers and only expose ICD symbols
for vulkan-loader which uses dlopen(3) to find the drivers via
${PREFIX}/share/vulkan/icd.d/*.json (or VK_ICD_FILENAMES).

Approved by: manu (on dev-commits-ports-main@ list)