common/mlx5: fix link with ibverbs glue dlopen option

common/mlx5: fix link with ibverbs glue dlopen option

In case the ibverbs glue is a separate library to dlopen,
the PMD library must allocate a glue structure to be filled by dlopen.

The glue management was in mlx5_common.c and moved to mlx5_common_os.c,
but the variable allocation was not removed from the original file.
The consequence was a link failure, if ibverbs dlopen option is enabled,
because of the redefinition of the variable (with GCC 10):
	multiple definition of 'mlx5_glue'

The original definition is removed to keep only the one moved
in the Linux sub-directory.

Fixes: 79aa430721b1 ("common/mlx5: split common file under Linux directory")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Matan Azrad <matan@mellanox.com>