Fix: remove NULL check of nonnull parameter in dlclose

The `handle` parameter of `dlclose`, as defined in
`include/lttng/ust-dlfcn.h` is marked `__nonnull`. GCC starting with
version 6.1 emits warnings for comparisons of nonnull arguments
against NULL by default (see -Wnonnull-compare).

Therefore, this removes a superfluous NULL check on `handle`, for
which GCC emitted a warning.

Signed-off-by: Antoine Busque <abusque@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>