Revert "test-functions: make sure we test our own libudev instead of the host libudev"

This reverts commit 73484ecf.

3976f372 moved libudev.so to be built in the
main directory, so this addition to $LD_LIBRARY_PATH is now obsolete.

After that commit, we build the following shared libraries:

build/libnss_myhostname.so.2
build/libnss_mymachines.so.2
build/libnss_resolve.so.2
build/libnss_systemd.so.2
build/libsystemd.so.0.30.0
build/libudev.so.1.7.0
build/pam_systemd.so
build/pam_systemd_home.so
build/src/boot/efi/stub.so
build/src/boot/efi/systemd_boot.so
build/src/shared/libsystemd-shared-247.so

EFI stubs don't matter, and libsystemd-shared-nnn.so is loaded through rpath,
and is doesn't need to and shouldn't be in $LD_LIBRARY_PATH. In effect, we only
ever need to add the main build directory to the search path.