ci: remove prepack and prepublishonly scripts (#14832)

This PR removes the `prepack` and `prepublishonly` scripts, which have
become unnecessary and counterproductive.

PR https://github.com/payloadcms/payload/pull/14820 introduced a new
`pnpm build:debug` command. However, when running our pre-release
publish workflow, each package is currently built twice: once via `pnpm
build:debug` (as intended) and again via the hard-coded `prepublishonly`
script. Because prepublishonly always runs a clean, non-debug build, it
both nullifies the debug output and adds significant overhead by
rebuilding everything.

All of our release and pack-to-destination scripts already perform a
full build ahead of time, so these additional prepublish hooks provide
no benefit and actively interfere with the intended debug workflow. This
PR removes the redundant scripts to ensure debug builds work as expected
and to speed up the release process.