maint: don't specify XZ_OPT=-9ev in dist-related rule

maint: don't specify XZ_OPT=-9ev in dist-related rule

Using xz's -9 option is warranted only if you have a very large
tarball (see xz's documentation for the sizes vs. presets), and
requires 64MiB of memory at decompression time.
* top/maint.mk (alpha beta stable): Don't specify XZ_OPT=-9ev.
Automake's default of just "-e" is fine.  Override on a
per-package basis by setting XZ_OPT e.g., in cfg.mk.