Removed all overrides to trend line display in plots; SUPREMM module

[jsperhac]

Trend line display default is now 'off' for all GroupBy classes, thus for all Usage plots, for supremm module. See also pull requests ubccr/xdmod-xsede#1 and ubccr/xdmod#2 which address this change for xdmod xsede and supremm modules.

Description

Overrides to getDefaultShowTrendLine() removed in all GroupBy classes; retained in parent GroupBy class.

Motivation and Context

Assigned as follows:
https://app.asana.com/0/14787510600562/217118195806574

Tests performed

Changes affect only Usage plots. Verified that Usage line plot types that previously overrode trend line behavior now display no trend lines by default, namely:
Jobs GroupByNSFStatus, and
GroupByNone for:

• Jobs
• Supremm
• Accounts
• Allocations
• Requests
• ResourceAllocations
• Grants
• Performance
• Proposal
  (note that the latter three plot types do not exist, in fact)

Verified that function definition for getDefaultShowTrendLine() is found only in parent GroupBy class, Query/GroupBy.php

Tests were done against clean merged code with xdmod6.5.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project as found in the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

…shown by default.