Remove deprecated LGTM badge in README.md. (#13895)

Why I did it
LGTM is deprecated. LGTM's badge doesn't work now.
Github code scanning shows alerts in Security tab. It doesn't have a badge.

How I did it
Remove LGTM badge.

How to verify it

README.md

@@ -1,8 +1,3 @@
-*static analysis*:
-
-[![Total alerts](https://img.shields.io/lgtm/alerts/g/Azure/sonic-buildimage.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/Azure/sonic-buildimage/alerts/)
-[![Language grade: Python](https://img.shields.io/lgtm/grade/python/g/Azure/sonic-buildimage.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/Azure/sonic-buildimage/context:python)
-
 *master builds*:
 
 [![Barefoot](https://dev.azure.com/mssonic/build/_apis/build/status/barefoot/Azure.sonic-buildimage.official.barefoot?branchName=master&label=Barefoot)](https://dev.azure.com/mssonic/build/_build/latest?definitionId=146&branchName=master)