#1096: simplified `findExistingForSocialLogin` finder #538

Job	Run time
Coding Standard & Static Analysis	24s
testsuite (8.1, sqlite)	1m 5s
testsuite (8.1, mysql)	1m 19s
testsuite (8.1, pgsql)	52s
testsuite (8.2, sqlite)	19s
testsuite (8.2, mysql)	32s
testsuite (8.2, pgsql)	27s
testsuite (8.3, sqlite)	27s
testsuite (8.3, mysql)	36s
testsuite (8.3, pgsql)	27s
	6m 28s

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