To override inside a vendor module, the layout should be placed in
app/design/frontend/mytheme/theme/Magento_Catalog/layout/catalog_category_view.xml
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<referenceContainer name="page.main.title" remove="true"/>
</body>
</page>
Make sure you clear your cache etc by running:
bin/magento s:d:c
bin/magento c:c