drop DATE from iga_version_string

[tjaalton]

Having the date there makes the build fail with the default distro build options:

/home/tjaalton/src/pkg-opencl/intel-graphics-compiler.git/visa/iga/IGALibrary/api/iga.cpp:620:62: error: macro "DATE" might prevent reproducible builds [-Werror=date-time]
return IGA_VERSION_STRING "-" IGA_EXTERNAL_INTERNAL " (" DATE ")";
^~~~~~~~

[paigeale]

Fix for this will be merged today

[paigeale]

Closing for issue is addressed with 670b902