`SASL_PATH` no longer has any effect but is still documented

[flowerysong]

In 79827fb (part of #745) SASL_PLUGINDIR replaced SASL_PATH as the environment variable used to configure the plugin directory at runtime. It's not clear that this was a deliberate change or what motivated it, since it's not mentioned in the commit message.

SASL_PATH is still referenced multiple times in the documentation, e.g.

cyrus-sasl/docsrc/sasl/installation.rst

Line 44 in 537af16

Note that the library can use the environment variable SASL_PATH to locate the directory where the mechanisms are; this should be a colon-separated list of directories containing plugins. Otherwise it will default to the value of `--with-plugindir` as supplied to `configure` (which itself defaults to `/usr/local/lib`).

and

cyrus-sasl/docsrc/sasl/quickstart.rst

Line 61 in 537af16

Note that the library can use the environment variable SASL_PATH to locate the

It also still has a vestigial presence in configure.ac, though nothing references this definition:

cyrus-sasl/configure.ac

Line 1391 in 537af16

#define SASL_PATH_ENV_VAR "SASL_PATH"