Chromium's Linux toolchain doesn't implement the return value of
std::vector::insert correctly, causing a build failure. Remove an
unnecessary assignment of the return value to fix this build regression.

The regression was caused by the commit:
"Put each array declarator into a separate declaration in HLSL output"

TEST=angle_unittests

Change-Id: I2959122d28e4c7e6d6a4d842a97a8f1fd4f27ca1
Reviewed-on: https://chromium-review.googlesource.com/266990Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>