PHP memory should be unlimited in scripts that need more memory than the default

[YongBoLiu]

Describe the bug
A clear and concise description of what the bug is.

The hard code of set memory_limit need to be removed. The hard code make user can't change the memory limit value in /etc/php.ini. And lead to errors as below,

ERROR PHP ERROR: Allowed memory size of 536870912 bytes exhausted

To Reproduce
Steps to reproduce the behavior:

1. Monitoring a large number of devices with more 354k graphs.
2. Check the Clog.

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.
The hard code is not good solution, and make the value in /etc/php.ini not work. The lines of below, need to be removed.

[yboliu@oc8636837557 cacti]$ grep -R ini_set.*memory_limit . |grep -Ev plugin|grep -Ev '\-1'|grep -Ev read_config_option |grep memory_limit
./poller_commands.php:ini_set('memory_limit', '800M');
./install/cli_check.php:ini_set('memory_limit','256M');
./poller_spikekill.php:ini_set('memory_limit', '512M');
./include/vendor/phpsnmp/mib_parser.php:		ini_set('memory_limit', '256M');
./cmd.php:ini_set('memory_limit', '512M');
./poller.php:ini_set('memory_limit', '512M');
./lib/import.php:		ini_set('memory_limit', '384M');

[netniV]

My only concern with this is that it will allow PHP to consume all available memory which means that the CMD.PHP and POLLER.PHP could both invoke a bad user script that eats the resources.

[rprofijt]

We have this patched locally because we have a plug-in that exceeds the amount in poller.php, this was very confusing because in my opinion the limit should be set in php.ini, I'm glad with this change :)