Revert "Add String.IndexOf to well known functions (#5342)" (#5362)
Revert "Add String.IndexOf to well known functions (#5342)" (#5362)

This reverts commit fe0b082321f3f3cb5bd64a6ccd324e24added884.

Fixes #5360.
cdmihai/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,mono/msbuild,AndyGerlicher/msbuild,cdmihai/msbuild,mono/msbuild,mono/msbuild,rainersigwald/msbuild,rainersigwald/msbuild,cdmihai/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,mono/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,sean-gilliam/msbuild