[w3c/manifest] Remove "steps to programmatically determine the directionality of a member" (#742)

Following up from https://github.com/w3c/manifest/issues/676#issuecomment-442658368, this seems to be unnecessary.

We currently have this normative text:

> When the base direction is "`auto`" the user agent *MUST* run the steps to programmatically determine the directionality of a member - and use the resulting text-direction value to assist in displaying the value of the member.
>
> The **steps to programmatically determine the directionality of a member** are as follows. The algorithm takes the *value* of a member.
>
> 1. Find the first character (in logical order) of the *value* that is of bidirectional character type L, AL, or R [[BIDI](https://www.unicode.org/reports/tr9/tr9-37.html)].
> 2. If such a character is found and it is of bidirectional character type AL or R, return "`rtl`".
> 3. Otherwise, return "`ltr`".

This seems entirely redundant for the text in [BIDI](http://unicode.org/reports/tr9/#P2):

> P2. In each paragraph, find the first character of type L, AL, or R while skipping over any characters between an isolate initiator and its matching PDI or, if it has no matching PDI, the end of the paragraph.
>
> ...
>
> P3. If a character is found in P2 and it is of type AL or R, then set the paragraph embedding level to one; otherwise, set it to zero.
>
> Whenever a higher-level protocol specifies the paragraph level, rules P2 and P3 may be overridden

Manifest is overriding rules P2 and P3, legitimately when `dir` is "`left`" or "`right`", but when `dir` is "`auto`", we unnecessarily override it with almost the exact same rule, only less robust (since we do not take isolates into account).

I suggest we drop this text and simply say that we only override BIDI when `dir` is "`left`" or "`right`".

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