Remove unused variable causing `tosa.variable` serialization to crash

This commit removes an unused variable that tried to get a result from
`tosa.variable`, but `tosa.variable` does not return a result. Also added
a test to prevent regression in the future.

Change-Id: Ia2ac521d0a5e7df645b3cb5c3a0af8a0ed844d18
Signed-off-by: Luke Hutton <luke.hutton@arm.com>