Revert "shared_lib: add as_needed to the libspdk.so linker script"

This reverts commit 346fefc3.
That commit breaks the shared object build when linking against the
implicit top level linker script. It makes it so that none of the
constructor functions get properly linked in.

Change-Id: I3477acbd86f44b6d5e949eb6725ca4c1b71a55b5
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/462149


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>