CONTRIBUTING.md: remove incorrect info about PKG_VERSION

[wehagy]

Description

The documentation stated that PKG_VERSION is the OpenWrt version and therefore includes the PKG_RELEASE, but this statement was incorrect and has been removed to avoid confusion.

Motivation and Context

Recently I have done a test commit 22e430c for netbird package, to write this I see others examples and read:

packages/CONTRIBUTING.md

Lines 200 to 205 in 86ad54b

	Makefile. The script is executed with the two arguments `PKG_NAME` and
	`PKG_VERSION`. The `PKG_NAME` can be used to distinguish package variants, e.g.
	`foobar` vs. `foobar-full`. The `PKG_VERSION` can be used for a trivial test
	checking if `foobar --version` prints the correct version. `PKG_VERSION` is the
	OpenWrt version and therefore includes the `PKG_RELEASE`, which isn't usually
	part of the running programs version.

The confusion is, the user can use PKG_VERSION to test the version of program, but at same time PKG_VERSION includes PKG_RELEASE and can't be used to test version?

Let's see how the contributors write their tests:

packages/utils/yq/test.sh

Line 3 in 86ad54b

yq --version 2>&1 | grep "$PKG_VERSION"

packages/net/sscep/test.sh

Line 3 in 86ad54b

sscep | grep -q "$PKG_VERSION"

packages/net/v2raya/test.sh

Line 3 in 86ad54b

v2raya --version | grep "$PKG_VERSION"

To be sure I dive in a little bit and see this:

packages/.github/workflows/entrypoint.sh

Lines 22 to 24 in 86ad54b

	# package version without release
	PKG_VERSION=$(sed -ne 's#^Version: \(.*\)$#\1#p' ./control)
	PKG_VERSION="${PKG_VERSION%-[!-]*}"

[wehagy]

Have some way to disable build test for things like this PR?