remove crd subnet/ippool deprecated property vlan default value

[Icarus9913]

Signed-off-by: Icarus9913 icaruswu66@qq.com

What type of PR is this?

new-feature, enhancement

What this PR does / why we need it:
remove crd subnet/ippool deprecated property vlan default value, so that I would not notice the vlan: 0 in the IPPool/Subnet resource

Which issue(s) this PR fixes:
close #2573

[codecov]

Codecov Report

Merging #2576 (aa1037c) into main (5c3a444) will increase coverage by 0.50%.
Report is 4 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2576      +/-   ##
==========================================
+ Coverage   80.87%   81.38%   +0.50%     
==========================================
  Files          49       49              
  Lines        5303     5303              
==========================================
+ Hits         4289     4316      +27     
+ Misses        857      826      -31     
- Partials      157      161       +4     

Flag	Coverage Δ
unittests	81.38% <100.00%> (+0.50%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
pkg/subnetmanager/subnet_manager.go	41.48% <100.00%> (ø)

... and 2 files with indirect coverage changes