SERVER-9221: Fixed broken linkage of libmongoclient.so when using --use-sasl-client and --sharedclient

The linkage of libmongoclient.so is broken, I applied the following fixes:
- gsasl was not linked against the lib when using --use-sasl-client and --sharedclient, added gsasl to the client LIBS
- two unneeded cpp files caused linker errors, removed them from the client sources (seems not to harm)
- only use clientEnv in src/SConscript.client (seems to be reasonable)

If you disagree with any of the changes, I would be glad to provide  a better fix for the problem.

This is related to:
https://jira.mongodb.org/browse/SERVER-9221