Remove various dead code

Remove various dead code

utils_alios.h had an unnecessary assignment

yallocbufferhandler.h had an unused member

gralloc1bufferhandler.h had an unused member

iahwc2.cpp and .h had a color_mode_ that was not used anywhere

gbmbufferhandler.cpp needs a fixme note since I am not sure why
the mod variable is being used outside of the if statement when it
isn't initialized to begin with.

Jira: None
Test: HWC build passes without errors
Signed-off-by: Richard Avelar <richard.avelar@intel.com>