Erase a removed pack from ResourcePack::<anonymous namespace>::packs
instead of only deleting the pack file from the disk.

This causes the repopulated table to actually display that the pack has
been removed (which previously only happened after restarting Dolphin or
hitting `Refresh`), and also removes the pack from the `Packs.ini` file.

Avoid accessing destroyed `QTableWidgetItem` by retrieving the desired
path from the item before it's destroyed.

In `ResourcePackManager::Remove` pointers to the selected items in `m_table_widget`
were saved in the local variable `items` before calling `Uninstall`.
`Uninstall` called `RepopulateTable` which called
`m_table_widget->clear()`, destroying the table's descendants.  Upon
returning to `Remove` `items` then pointed to some of those destroyed descendants, and
passing `items[0]` to `GetResourcePackIndex` resulted in a call to
`item->row()` which was a use-after-free.

Fixes https://bugs.dolphin-emu.org/issues/14095.

Remove window hints clearing the flag Qt::WindowContextHelpButtonHint,
which is already off by default in Qt 6.

In Qt 5 this flag was set by default for QDialogs, and on Windows put a
? button in the corner of the title bar allowing users to activate Qt's
QWhatsThis help system for a given widget. Since we don't set that text
the ? button was useless and so we hid it manually.

This is text that should be translated, since it'll show up as English
for non-English speakers.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.