C2SoftGav1Dec: remove non-eos flush

eos is later checked and signalled in drainInternal. signalling eos on a
zero sized frame will cause subsequent frame decodes to fail until the
next keyframe.

fixes:
Codec2VideoDecHidlTest#DecodeTestEmptyBuffersInserted/software_c2_android_av1_decoder_0
after ag/12144538 (Idc713f4b10776cfb6b7fa2d75b2162c3b85d3313)

Bug: 162437379
Bug: 162321377
Bug: 162267932
Test: vts-tradefed run commandAndExit vts --include-filter VtsHalMediaC2V1_0TargetVideoDecTest
Change-Id: Ied4cd9b6ad56f3630d9779ea32d0032a5bda1a29