Now that we can build a bdb-only depends prefix, there is no need to
maintain a bdb-building bash script, that does the same things as
depends, except worse, as it's missing patches and workarounds. i.e bitcoin#26623.

44f3c7d contrib: remove install_db4.sh (fanquake)
14ce843 doc: add new NO_* options from #26833 (fanquake)

Pull request description:

  Now that we can build a bdb-only depends prefix (#26833), there is no need to
  maintain a bdb-building bash script, that does the same thing as
  depends, except worse, as it's missing patches and workarounds. i.e #26623.

  Someone that wants to compile bdb themselves, but doesn't want to use other depends built libs, can do:
  ```bash
  make -C depends NO_BOOST=1 NO_LIBEVENT=1 NO_QT=1 NO_SQLITE=1 NO_NATPMP=1 NO_UPNP=1 NO_ZMQ=1 NO_USDT=1
  ...
  to: /path/to/bitcoin/depends/x86_64-pc-linux-gnu
  ```

  which gives them a BDB only prefix, and then compile using:
  ```bash
  export BDB_PREFIX="/path/to/bitcoin/depends/x86_64-pc-linux-gnu"
  ./autogen.sh
  ./configure \
      BDB_LIBS="-L${BDB_PREFIX}/lib -ldb_cxx-4.8" \
      BDB_CFLAGS="-I${BDB_PREFIX}/include"
  ```

  Wondering if we should extract the build bdb/legacy wallet docs somewhere, to avoid the repetition?

ACKs for top commit:
  TheCharlatan:
    ACK 44f3c7d
  achow101:
    ACK 44f3c7d
  hebasto:
    ACK 44f3c7d
  jarolrod:
    ACK 44f3c7d

Tree-SHA512: 50b33ae9df2ab94a1bd114e846cec16f647a61023b72f0d3e547a18db09c01d60bb7b42a04758212f4930314df03016feb6ebc96962dd8a8e26eb8cd4e0d167d