Delete the plugin-related text in the cargo-target document.

src/doc/src/reference/cargo-targets.md

@@ -184,7 +184,6 @@ test = true            # Is tested by default.
 doctest = true         # Documentation examples are tested by default.
 bench = true           # Is benchmarked by default.
 doc = true             # Is documented by default.
-plugin = false         # This option is deprecated and unused.
 proc-macro = false     # Set to `true` for a proc-macro library.
 harness = true         # Use libtest harness.
 edition = "2015"       # The edition of the target.
@@ -245,10 +244,6 @@ libraries and binaries.
 > **Note**: The binary will be skipped if its name is the same as the lib
 > target.
 
-### The `plugin` field
-
-This field is used for `rustc` plugins, which are being deprecated.
-
 ### The `proc-macro` field
 
 The `proc-macro` field indicates that the library is a [procedural macro]