[NFCI] Remove unused variables and enable -Wunused-variable (#1483)

Remove unused variables throughout the code base and enable the
`-Wunused-variable` warning flag globally to prevent new unused
variable issues being introduced in the future.

This is mostly a non-functional change, with one exception:

 - In `test_conformance/api/test_kernel_arg_info.cpp`, an error check
   of the clGetDeviceInfo return value was added.

Signed-off-by: Sven van Haastregt <sven.vanhaastregt@arm.com>

test_conformance/pipes/test_pipe_read_write.cpp

@@ -626,7 +626,6 @@ int test_pipe_readwrite_struct_generic( cl_device_id deviceID, cl_context contex
     size_t size = sizeof(TestStruct);
     size_t global_work_size[3];
     cl_int err;
-    int total_errors = 0;
     int i;
     MTdataHolder d(gRandomSeed);
     clEventWrapper producer_sync_event = NULL;