chore: remove useless val

[daixiang0]

What type of PR is this?

Now we use IMAGE_PLATFORMS rather than IMAGE_PLAT

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Release Notes: Yes/No

[daixiang0]

root:[gateway]$ git grep IMAGE_PLAT
tools/make/common.mk:53:        IMAGE_PLAT := linux_$(GOARCH)
tools/make/common.mk:57:        IMAGE_PLAT := $(PLATFORM)
tools/make/image.mk:14:IMAGE_PLATFORMS ?= linux_amd64 linux_arm64
tools/make/image.mk:18:$(eval BUILDX_PLATFORMS = $(shell echo "${IMAGE_PLATFORMS}" | sed "s# #,#;s#_#/#g"))

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.29%. Comparing base (52d8f9c) to head (5720b65).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5644      +/-   ##
==========================================
+ Coverage   65.27%   65.29%   +0.01%     
==========================================
  Files         213      213              
  Lines       34111    34111              
==========================================
+ Hits        22266    22272       +6     
+ Misses      10505    10499       -6     
  Partials     1340     1340              

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