Remove unused NVP_EXT_PATH variable

The NVP_EXT_PATH variable has been renamed to NSX_EXT_PATH and
it's now imported directly from the vmware module in the __init__
of the NsxPluginV2 driver

Change-Id: I68661ec25bf730caf8a9910a26c82cb647ace750
Signed-off-by: Rosario Di Somma <rosario.disomma@dreamhost.com>