[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/api/test_mem_object_info.cpp

@@ -363,8 +363,6 @@ int test_get_imageObject_info( cl_mem * image, cl_mem_flags objectFlags, cl_imag
     cl_mem_flags flags;
     cl_uint mapCount;
     cl_uint refCount;
-    size_t rowPitchMultiplier;
-    size_t slicePitchMultiplier;
     cl_context otherCtx;
     size_t offset;
     size_t sz;