Hi there,
Just want to start by saying it is important to create a child theme before you make the following changes. With that said, the best way to achieve what you are doing is to remove the lines from the page templates themselves.
Since you are only using the full-width template, then your only change would be to line 18 in content-page.php, or you may want to remove lines 17-19 if you don’t want the underline to appear above the content.
<header class="entry-header">
<?php the_title( '<h1 class="entry-title">', '</h1>' ); ?>
</header><!-- .entry-header -->
However, if you have a blog, or have a search, etc then you may need to update all of your content-*.php files as well. Here are just a few:
line 26 on content.php:
<?php the_title( sprintf( '<h1 class="entry-title"><a href="%s" rel="bookmark">', esc_url( get_permalink() ) ), '</a></h1>' ); ?>
line 16 on content-single.php:
<?php the_title( '<h1 class="entry-title">', '</h1>' ); ?>
line 13 in content-search.php:
<?php the_title( sprintf( '<h1 class="entry-title"><a href="%s" rel="bookmark">', esc_url( get_permalink() ) ), '</a></h1>' ); ?>
Hope that helps!