Remove read value check against uncommitted pages

Remove read value check against uncommitted pages

Reading from uncommitted pages produces undefined results so checks that
the result is zero in invalid. Only data read from committed pages has
undefined results. The PixelUnpackBufferStorageTestCase subtest was the
only subtest that was checking reads from uncommitted pages. All the
other subtest already did not do this.

Affects:
KHR-GL46.sparse_buffer_tests.BufferStorageTest

Components: OpenGL

VK-GL-CTS Issue: 921

Change-Id: Ie7e61553cbad2454af03f39b7cdbd1ae8ab66d93