Remove log4j from H2O example

[adriangonz]

The pom.xml of the h2o_mojo example model installs a log4j version, which has recently been flagged as insecure.

seldon-core/examples/models/h2o_mojo/pom.xml

Lines 60 to 64 in 62766f6

	<dependency>
	<groupId>log4j</groupId>
	<artifactId>log4j</artifactId>
	<version>1.2.17</version>
	</dependency>

Checking out the code, it seems that it's using SLF4J instead, therefore I can't see any reason for log4j to be installed. It should be safe to just remove it.