fix(copy): Remove some redundant text in the beegfs copy usage message

[Willard84]

What does this PR do / why do we need it?

beegfs copy --help showed the default value for --chunksize and --partition-threshold twice: once in the flag's own description text, once via pflag's auto-generated (default N) suffix. Suppresses the redundant suffix by setting each flag's DefValue to "0", which trips pflag's existing defaultIsZeroValue() check (see spf13/pflag's flag.go) that skips printing (default N) when the default is the type's zero value. The actual bound default (128MB / 1024MB) is unaffected -- DefValue is a display-only string, separate from the flag's real value.

Follows the same pattern already used in ctl/internal/cmd/rst/pushpull.go for PriorityFlag.

Verified: built and ran beegfs copy --help -- output now shows only the explicit "Default chunk size: 128 MB." text, no more duplicate (default 128). Confirmed via gofmt and go build/go vet (GOOS=linux) that nothing else broke.

Related Issue(s)

#323

Where should the reviewer(s) start reviewing this?

Single 5-line change in ctl/internal/cmd/copy/copy.go.

Are there any specific topics we should discuss before merging?

Not required.

What are the next steps after this PR?

Not required.

Checklist before merging:

• Documentation: no developer or user documentation changes needed beyond the usage message itself (which this PR fixes).
• Testing: cosmetic help-text change; no new tests needed.
• Git Hygiene: single commit, Conventional Commits format.