[llvm] [llvm][tools][opt-viewer] Remove Python2 compatability code in optrecord.py (PR #163744)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ml-opt-dev-x86-64` running on `ml-opt-dev-x86-64-b1` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/137/builds/28145

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: tools/opt-viewer/filter.test' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 5
/usr/bin/python3 /b/ml-opt-dev-x86-64-b1/llvm-project/llvm/tools/opt-viewer/opt-viewer.py --filter inline -s /b/ml-opt-dev-x86-64-b1/llvm-project/llvm/test/tools/opt-viewer/Inputs -o /b/ml-opt-dev-x86-64-b1/build/test/tools/opt-viewer/Output/filter.test.tmp /b/ml-opt-dev-x86-64-b1/llvm-project/llvm/test/tools/opt-viewer/Inputs/basic/or.yaml --no-highlight --demangler=/b/ml-opt-dev-x86-64-b1/build/bin/llvm-cxxfilt
# executed command: /usr/bin/python3 /b/ml-opt-dev-x86-64-b1/llvm-project/llvm/tools/opt-viewer/opt-viewer.py --filter inline -s /b/ml-opt-dev-x86-64-b1/llvm-project/llvm/test/tools/opt-viewer/Inputs -o /b/ml-opt-dev-x86-64-b1/build/test/tools/opt-viewer/Output/filter.test.tmp /b/ml-opt-dev-x86-64-b1/llvm-project/llvm/test/tools/opt-viewer/Inputs/basic/or.yaml --no-highlight --demangler=/b/ml-opt-dev-x86-64-b1/build/bin/llvm-cxxfilt
# .---command stdout------------
# | Reading YAML files...
# | 
# | 	1 of 1
# `-----------------------------
# .---command stderr------------
# | Traceback (most recent call last):
# |   File "/b/ml-opt-dev-x86-64-b1/llvm-project/llvm/tools/opt-viewer/opt-viewer.py", line 471, in <module>
# |     main()
# |   File "/b/ml-opt-dev-x86-64-b1/llvm-project/llvm/tools/opt-viewer/opt-viewer.py", line 455, in main
# |     map_remarks(all_remarks)
# |   File "/b/ml-opt-dev-x86-64-b1/llvm-project/llvm/tools/opt-viewer/opt-viewer.py", line 299, in map_remarks
# |     for remark in optrecord.itervalues(all_remarks):
# |                   ^^^^^^^^^^^^^^^^^^^^
# | AttributeError: module 'optrecord' has no attribute 'itervalues'
# `-----------------------------
# error: command failed with exit status: 1

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/163744