release: remove default from VERSION_ID

[zyga]

According to os-release(5) there is no default value for this field so
we should not provide one. This also removes the unexpected "unknown"
element from "snap version" on rolling releases, such as Arch.

Signed-off-by: Zygmunt Krynicki me@zygoon.pl

[codecov-io]

Codecov Report

Merging #3648 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3648      +/-   ##
==========================================
+ Coverage    75.2%   75.21%   +<.01%     
==========================================
  Files         387      387              
  Lines       33452    33451       -1     
==========================================
+ Hits        25159    25160       +1     
+ Misses       6481     6479       -2     
  Partials     1812     1812

Impacted Files	Coverage Δ
release/release.go	89.18% <ø> (-0.15%)	⬇️
overlord/ifacestate/helpers.go	63% <0%> (+0.66%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e037060...81bd765. Read the comment docs.