Remove unused "PC STACK" variables for from the left collectors (this breaks compatibility with polycyclic version 2.16 and older; use 2.17 or newer instead)

[stertooy]

Some minor cleanup. Removes unused variables that were originally used by Polycyclic (see gap-packages/polycyclic#6).

Text for release notes

Removed some variables that were previously used by the Polycyclic package. This breaks compatibility with Polycyclic versions 2.16 and older, upgrading to version 2.17 or newer is required to use this package.

Further details

This breaks compatibility with older versions of Polycyclic (i.e. 2.16 and older). Latest version (2.17) works fine.

[ChrisJefferson]

This is definitely a PR we want to merge at some point, the only question is if we should merge it now, or leave it open and give people a little bit longer to upgrade.

[fingolfin]

Then we should it do the GAPInfo.PackagesRestrictions record in the GAP library, which lists for several packages minimal required versions.

[stertooy]

This is definitely a PR we want to merge at some point, the only question is if we should merge it now, or leave it open and give people a little bit longer to upgrade.

There's certainly no rush in merging this. Though, given the number of bug fixes in Polycyclic 2.17, I don't think pushing users to upgrade is necessarily a bad thing.

A possible middle ground is to leave in the line

gap/src/objcftl.c

Line 414 in 7e76a45

AssConstantGVar(GVarName("NO_STACKS_INSIDE_COLLECTORS"), True);

Then Polycyclic 2.15, 2.15.1 and 2.16 will produce Syntax Warnings when loaded, but they will actually work fine.

Then we should it do the GAPInfo.PackagesRestrictions record in the GAP library, which lists for several packages minimal required versions.

Added to the PR :)

[fingolfin]

OK, it's fine to me as it is, with the warning, and for GAP 4.16.0.

But I suggest we put this into the release note then, with a suitable text informing users (for most it won't matter, but if someone runs into the error, then they can at least in principle find it in the NEWS)