[llvm-cov] Remove some asserts in the html renderer (NFC)

These asserts are making tests fragile. The renderer does not enter an
invalid state when they fail, however, it may spit out a garbled
coverage report because the source text no longer matches the provided
coverage mapping.

Another follow-up to r281072.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281076 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed