cleanup: remove type error suppression in webcomponents-externs

[stephanwlee]

let statements in https://github.com/Polymer/polymer/blob/2.x/externs/webcomponents-externs.js causes serious type error like below:

Jul 15, 2019 8:33:41 AM com.google.javascript.jscomp.LoggerErrorManager println
WARNING: external/org_polymer_externs/externs/webcomponents-externs.js:15: WARNING - [JSC_BLOCK_SCOPED_DECL_MULTIPLY_DECLARED_ERROR] Duplicate let / const / class / function declaration in the same scope is not allowed.
let HTMLImports = {
    ^^^^^^^^^^^^^^^

Jul 15, 2019 8:33:41 AM com.google.javascript.jscomp.LoggerErrorManager println
WARNING: external/org_polymer_externs/externs/webcomponents-externs.js:15: WARNING - [JSC_REDECLARED_VARIABLE_ERROR] Illegal redeclared variable: HTMLImports                         
let HTMLImports = {
    ^^^^^^^^^^^^^^^

Newer externs for (v3; internal version matches that of v3) seems to be okay.
https://github.com/Polymer/polymer/blob/master/externs/webcomponents-externs.js