[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/basic/test_hiloeo.cpp

@@ -43,8 +43,6 @@ static const unsigned int out_vector_idx[] =   { 0, 0, 1, 1, 3, 4};
 // input type name is strcat(gentype, vector_size_names[i]);
 // and output type name is
 // strcat(gentype, vector_size_names[out_vector_idx[i]]);
-static const int size_to_idx[] = {-1,0,1,2,3,-1,-1,-1,4,
-    -1,-1,-1,-1,-1,-1,-1,5};
 static const char *vector_size_names[] = { "", "2", "3", "4", "8", "16"};
 
 static const size_t  kSizes[] = { 1, 1, 2, 2, 4, 4, 8, 8, 4, 8 };