Cleanup unneeded lifetimes in bevy_asset (#15546)

# Objective

Fixes #15541

A bunch of lifetimes were added during the Assets V2 rework, but after
moving to async traits in #12550 they can be elided. That PR mentions
that this might be the case, but apparently it wasn't followed up on at
the time.

~~I ended up grepping for `<'a` and finding a similar case in
`bevy_reflect` which I also fixed.~~ (edit: that one was needed
apparently)

Note that elided lifetimes are unstable in `impl Trait`. If that gets
stabilized then we can elide even more.

## Solution

Remove the extra lifetimes.

## Testing

Everything still compiles. If I have messed something up there is a
small risk that some user code stops compiling, but all the examples
still work at least.

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## Migration Guide

The traits `AssetLoader`, `AssetSaver` and `Process` traits from
`bevy_asset` now use elided lifetimes. If you implement these then
remove the named lifetime.