in src/main/java/org/apache/sling/testing/mock/jcr/MockNode.java [580:591]
public void removeMixin(final String mixinName) throws RepositoryException {
if (this.hasProperty(JcrConstants.JCR_MIXINTYPES)) {
Value[] currentValues = getProperty(JcrConstants.JCR_MIXINTYPES).getValues();
Value valueToBeRemoved = this.getSession().getValueFactory().createValue(mixinName);
Value[] newValues = Arrays.stream(currentValues)
.filter(value -> !value.equals(valueToBeRemoved))
.toArray(Value[]::new);
this.setProperty(JcrConstants.JCR_MIXINTYPES, newValues);
} else {
throw new NoSuchNodeTypeException("Cannot remove blank mixin");
}
}