Remove global status counter Innodb_pages0_read

MDEV-9931 introduced a counter for keeping track of reads of the first page of InnoDB data files, because the original implementation of data-at-rest-encryption for InnoDB introduced new code paths for reading the pages.

Ultimately, the extra reads of the first page were removed, and the encryption subsystem will be initialized whenever we first read the first page of each data file, in fil_node_open_file(). It should not be that interesting to observe how many times an InnoDB data file was opened for the first time.