Remove manual clamping code in `src/core/jpx.js`
Remove manual clamping code in `src/core/jpx.js`

Since we're now using `Uint8ClampedArray`, rather than `Uint8Array`, doing manual clamping shouldn't be necessary given that that is now handled natively.

This shouldn't have any measurable performance impact, but just to sanity check that I've done some quick benchmarking with the following manifest file:
```json
[
    {  "id": "S2-eq",
       "file": "pdfs/S2.pdf",
       "md5": "d0b6137846df6e0fe058f234a87fb588",
       "rounds": 100,
       "type": "eq"
    }
]
```
which gave the following results against the current `master` (repeated benchmark runs didn't result in any meaningful differences):
```
-- Grouped By browser, stat --
browser | stat         | Count | Baseline(ms) | Current(ms) | +/- |    %  | Result(P<.05)
------- | ------------ | ----- | ------------ | ----------- | --- | ----- | -------------
firefox | Overall      |   100 |          592 |         592 |   1 |  0.12 |
firefox | Page Request |   100 |            3 |           3 |   0 | -9.88 |
firefox | Rendering    |   100 |          588 |         589 |   1 |  0.18 |
```

mukulmishra18/pdf.js,Snuffleupagus/pdf.js,showpad/mozilla-pdf.js,Rob--W/pdf.js,caremerge/pdf.js,mozilla/pdf.js,nawawi/pdf.js,Rob--W/pdf.js,Shoobx/pdf.js,mainegreen/pdf.js,mozilla/pdf.js,Shoobx/pdf.js,adlerweb/pdf.js-Touch,macroplant/pdf.js,Shoobx/pdf.js,xavier114fch/pdf.js,mainegreen/pdf.js,caremerge/pdf.js,Snuffleupagus/pdf.js,mozilla/pdf.js,nawawi/pdf.js,xavier114fch/pdf.js,websirnik/pdf.js,Snuffleupagus/pdf.js,timvandermeij/pdf.js,mukulmishra18/pdf.js,timvandermeij/pdf.js,macroplant/pdf.js,websirnik/pdf.js,nawawi/pdf.js,timvandermeij/pdf.js,skalnik/pdf.js,macroplant/pdf.js,adlerweb/pdf.js-Touch,showpad/mozilla-pdf.js,skalnik/pdf.js,showpad/mozilla-pdf.js