Remove libmongoc 1.15 from evergreen testing matrix
Remove libmongoc 1.15 from evergreen testing matrix

jmikola/mongo-php-driver-prototype,mongodb/mongo-php-driver,mongodb-labs/mongo-php-driver-prototype,mongodb/mongo-php-driver,mongodb-labs/mongo-php-driver-prototype,jmikola/mongo-php-driver-prototype,jmikola/mongo-php-driver-prototype,jmikola/mongo-php-driver-prototype,jmikola/mongo-php-driver,jmikola/mongo-php-driver,jmikola/mongo-php-driver-prototype,mongodb/mongo-php-driver,jmikola/mongo-php-driver,mongodb/mongo-php-driver,mongodb-labs/mongo-php-driver-prototype,mongodb-labs/mongo-php-driver-prototype,mongodb-labs/mongo-php-driver-prototype,jmikola/mongo-php-driver,jmikola/mongo-php-driver,mongodb/mongo-php-driver