Simplify d2i_PKCS7 by removing redundant BER-to-DER conversion

[justsmth]

Description of changes:

d2i_PKCS7 previously performed a manual BER-to-DER conversion using
CBS_asn1_ber_to_der before passing the result to ASN1_item_d2i. This
manual conversion had a bug: when BER input contained indefinite-length
elements, the converted DER encoding was shorter than the original BER, but
*in was advanced by the DER byte count rather than the original BER byte
count. This caused *in to be under-advanced, violating the d2i API contract.

Since ASN1_item_d2i already handles BER encoding natively (including
indefinite-length elements and constructed strings), the manual conversion
was redundant. This change removes it entirely and replaces
IMPLEMENT_ASN1_ALLOC_FUNCTIONS(PKCS7) plus the hand-written d2i_PKCS7
and i2d_PKCS7 with a single IMPLEMENT_ASN1_FUNCTIONS(PKCS7) macro call,
consistent with every other type defined in this file.

The now-unused #include "../bytestring/internal.h" has also been removed.

Call-outs:

The existing BER test in TestCertReparse was asserting the old (incorrect)
pointer-advancement behavior for BER input, expecting *in to advance by
only the DER-converted length. This has been updated to expect full
advancement past the original input, consistent with the non-BER path.

Testing:

All 60 existing PKCS7 tests pass (crypto_test --gtest_filter='*PKCS7*').

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.35%. Comparing base (40ec390) to head (15cae8f).
⚠️ Report is 58 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3037      +/-   ##
==========================================
- Coverage   78.36%   78.35%   -0.01%     
==========================================
  Files         689      689              
  Lines      121042   121022      -20     
  Branches    16967    16965       -2     
==========================================
- Hits        94851    94830      -21     
- Misses      25295    25296       +1     
  Partials      896      896              

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