EAMxx: use a single pack size cmake setting

[bartgol]

Other than SCREAM_PACK_SIZE we have SCREAM_SMALL_PACK_SIZE and SCREAM_POSSIBLY_NO_PACK_SIZE. The last two are never used. I mean, they are used to set the homonymous macros, but a) the macro SCREAM_POSSIBLY_NO_PACK_SIZE is never used in EAMxx, and b) our mach files never set SCREAM_SMALL_PACK_SIZE smaller than SCREAM_PACK_SIZE.

Supporting all these different options for no practical reason seems like a waste. The complexity of the code is larger, when we could get away with just one (app-wide) pack size. Of course, particular files can always something like

constexpr int MY_SMALL_PACK_SIZE = SCREAM_PACK_SIZE>1 ? SCREAM_PACK_SIZE / 2 : 1;

but keeping a cmake vars for something that is never configured differently from the default, seems pointless.

@jgfouca you were involved in the seminal pack work, so your opinion is very important here. Other devs can weigh in too, of course.