base-files: sysupgrade: fix streaming backup archives to stdout

base-files: sysupgrade: fix streaming backup archives to stdout

Due to previous refactoring in sysupgrade, writing backup archives to
stdout became impossible since the hardcoded gzip output redirection
did not account for the `-` special case filename.

Fix this issue by substituting `-` with `/proc/self/fd/1` in the tar
archive output path variable.

Also remove a redundant `rm -f` of the target file path that occurs
before the file could've possibly been written.

Fixes: #14773
Fixes: https://github.com/openwrt/luci/issues/6961
Fixes: e36cc53092 ("base-files: sysupgrade: use tar helper to include installed_packages.txt")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>