Fix auto provider detection

[Colengms]

We're seeing a scenario in which one provider will register, get automatically selected because it's the only one (and then provide a browse configuration), but not get properly un-selected when a subsequent provider registers.

Removed a check that was opportunistically using a configuration provider if registered and if we received a browse configuration for it, as this scenario would lead to the 'too early' automatically selected config provider being kept.

We have a pattern in place to clear out any modifications we had made to the configuration (by calling parsePropertiesFile again) when we need to reprocess it. But, that was a NO-OP if there was no c_cpp_properties.json file, so our modifications would not get removed in that scenario (making the prior registered configuration provider look like the user had explicitly specified it).  I updated that to instead use the default configuration, and I needed to fix one place where we didn't do that reset if the contents were empty.

[sean-mcmanus]

This breaks the 1 provider auto-select scenario (the 2 provider scenario is fixed). I see this.configuration.CurrentConfigurationProvider return '' instead of the 1 provider.