Fix bug in LUExportInstance

If remove_instance isn't passed to the Opcode, it'll be None.  When
generating the hooks environment, the error “int() argument must be a
string or a number” was raised. It should now behave as any other
boolean hook env variable.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>