fix(doc/lean/expr): remove obsolete documentation

It is subsumed by the Lean tutorial.

closes #431

doc/lean/expr.md

@@ -1,48 +0,0 @@
-# Expressions
-
-Lean is based on dependent type theory, and is very similar to the one
-used in the [Boole](https://github.com/avigad/boole) and
-[Coq](http://coq.inria.fr/) systems.  In contrast to Coq, Lean is
-classical.
-
-In Lean, we have the following kind of expressions: _constants_,
-,_function applications_, _(heterogeneous) equality_, _local variables_,
-_lambdas_, _dependent function spaces_ (aka _Pis_), _let expressions_,
-and _Types_.
-
-## Constants
-
-Constants are essentially references to variable declarations, definitions, axioms and theorems in the
-environment. In the following example, we use the command `variables` to declare `x` and `y` as integers.
-The `check` command displays the type of the given expression. The `x` and `y` in the `check` command
-are constants. They reference the objects declared using the command `variables`.
-
-```lean
-variables x y : Nat
-check x + y
-```
-
-In the following example, we define the constant `s` as the sum of `x` and `y` using the `definition` command.
-The `eval` command normalizes the expression `s + 1`. In this example, `eval` will just expand
-the definition of `s`, and return `x + y + 1`.
-
-```lean
-definition s := x + y
-eval s + 1
-```
-
-## Function applications
-
-In Lean, the expression `f t` is a function application, where `f` is a function that is applied to `t`.
-We define the function `double`
-
-```lean
-import tactic    -- load basic tactics such as 'simp'
-definition double (x : Nat) : Nat := x + x
-```
-
-In the following command, we define the function `inc`, and evaluate some expressions using `inc` and `max`.
-
-```lean
-definition inc (x : Nat) : Nat := x + 1
-```