Stop statically compiling the FTP extension (#1482)

* Stop statically compiling the FTP extension

According to the documentation of `ftp_ssl_connect()`, the limitation of
requiring a static build was lifted with PHP 7.0. Thus stop forcibly including
the FTP extension, which should be needed somewhat rarely.

Example:

    root@2a4c93db4a86:/var/www/html# apt-get update -qqqqq
    root@2a4c93db4a86:/var/www/html# apt-get install -yyyyqqqq libssl-dev
    debconf: delaying package configuration, since apt-utils is not installed
    Selecting previously unselected package libssl-dev:amd64.
    (Reading database ... 13257 files and directories currently installed.)
    Preparing to unpack .../libssl-dev_3.0.11-1~deb12u2_amd64.deb ...
    Unpacking libssl-dev:amd64 (3.0.11-1~deb12u2) ...
    Setting up libssl-dev:amd64 (3.0.11-1~deb12u2) ...
    root@2a4c93db4a86:/var/www/html# docker-php-ext-configure ftp --with-openssl-dir >/dev/null
    root@2a4c93db4a86:/var/www/html# docker-php-ext-install ftp >/dev/null
    + strip --strip-all modules/ftp.so
    root@2a4c93db4a86:/var/www/html# php -r "var_dump(function_exists('ftp_ssl_connect'));"
    bool(true)

* Drop `--enable-ftp` for the next patch

Author: TimWolla