tock-register-interface: remove warning about disabled offset tests

Removes warnings about disabled unit tests checking offsets and
alignments on `make ci-travis` from tock-register-interface's
README. These tests are indeed run on a `make ci-job-chips`.

As referenced in here: tock/tock#2618 (comment)

Author: hnj2

libraries/tock-register-interface/README.md

@@ -104,36 +104,6 @@ version = "0.4.x"
 features = ["no_std_unit_tests"]
 ```
 
-WARNING: For now, the **unit tests checking offsets and alignments are not yet
-run** on `make ci-travis`. This means that leaving an unintentional gap between
-registers will **not** be caught. Instead, the `register_structs` macro will
-generate a struct with invalid offsets without warning. Please follow the
-discussion on https://github.com/tock/tock/pull/1393.
-
-For example, the following call to the macro:
-
-```rust
-register_structs! {
-    Registers {
-        (0x000 => foo: ReadOnly<u8>),
-        (0x008 => bar: ReadOnly<u8>),
-        (0x009 => @END),
-    }
-}
-```
-
-will generate the following struct, even though there is an unintentional gap of
-4 bytes between addresses `0x004` (the end of register `foo`) and `0x008` (the
-intended beginning of register `bar`).
-
-```rust
-#[repr(C)]
-struct Registers {
-    foo: ReadOnly<u32>,
-    bar: ReadOnly<u32>,
-}
-```
-
 By default, the visibility of the generated structs and fields is private. You
 can make them public using the `pub` keyword, just before the struct name or the
 field identifier.